1. Field of the Invention
This invention generally relates to a beam mislanding correcting system for a color cathode ray tube, and more particularly is directed to an improvement in a system for compensating for the mislanding of electron beams in a cathode ray tube caused by thermal expansion of a beam selecting structure provided in the tube.
2. Description of the Prior Art
In a color cathode ray tube having a color phosphor screen, there is provided a beam selecting structure, in the form of a mask or grille electrode which has a plurality of apertures or slits, to have the electron beams land only on the selected phosphors of the screen that emit light of predetermined colors. During operation of such tubes, the impingement of the electron beams on the beam selecting structure generates heat and thereby increases the temperature of the beam selecting structure. The increased temperature of the beam selecting structure causes thermal expansion or distortion thereof, and such as thermal expansion or distortion changes the positions of the apertures or slits of the beam selecting structure in relation to the respective groups of color phosphors of the screen. The change in positions of the apertures or slits relative to the respective groups of color phosphors results in mislanding of the electron beams and this causes deterioration in color purity. The mislanding of the electron beams is more striking at the peripheral portions of the screen than at the center thereof.
Various systems have been proposed to compensate for the described mislanding of the electron beams resulting from thermal expansion of the beam selecting structure. A co-pending application, Ser. No. 451,306, filed Mar. 14, 1974 now U.S. Pat. No. 3,950,671, and assigned to the assignee of the present application, discloses a novel system for compensating for the mislanding of the electron beams in such a way as to avoid some of the drawbacks inherent in existing systems.
In the system of the above mentioned co-pending application, compensation for thermally induced mislanding of the electron beams is provided by electromagnetic means, for example, a coil wound on a magnetic core, disposed adjacent the color cathode ray tube and being operative, when supplied with current, to produce magnetic fields which change the paths of the electron beams passing through the beam selecting structure, and the current for such a coil is controlled in accordance with the temperature of the beam selecting means. In practice, since it is quite difficult to directly measure the temperature of the beam selecting structure, the temperature of something which undergoes temperature changes similar to those of the beam selecting structure is detected by a temperature responsive element connected to the circuit for supplying the current to the coil and thereby the current varying in accordance with changes in the temperature of the beam selecting structure is supplied to the coil. In such a case, generally, a horizontal and vertical deflection yoke is taken as the thing undergoing temperature changes similar to those of the beam selecting structure. However, although the temperature of the beam selecting structure increases with the result that thermal expansion or distortion thereof is caused immediately after the start of operation of the television receiver, the temperature of the deflection yoke is delayed to start changing similarly to those of the beam selecting structure. Consequently, during this delay in the start of temperature change, the current supplied to the coil is not controlled properly and the mislanding of the electron beam can not be compensated for. The length of duration of such a delay depends on the respective television receivers and may be one to ten minutes in general.